The technology of this disclosure relates to measurement of optical loss in optical fibers using optical time domain reflectometry. The disclosure is particularly related to calibration of optical loss measurement in optical time domain reflectometry for joined optical fibers having potentially dissimilar properties, resulting in the joint optical fibers having potentially dissimilar light backscattering efficiencies, since collection efficiencies may be caused by a variety of different optical properties, as discussed below.
Benefits of utilizing optical fiber include extremely wide bandwidth and low noise operation. Because of these advantages, optical fiber is increasingly being used for a variety of communications applications. These communications applications include, but are not limited to, broadband voice, video, and data transmissions in communications networks.
It is desirable to be aware of optical loss (i.e., optical attenuation) present in optical fiber used in communications networks to understand its impact on communications performance. Loss in optical fiber (referred to as “optical fiber loss”) can cause signal loss, which can reduce the bandwidth and data rate capabilities of the optical fiber. Optical fiber loss is the reduction in light intensity between light transmitted from an input of the optical fiber to an output of the optical fiber. Optical fiber loss is usually expressed as decibels (dB) per kilometer (km) (dB/km). Optical fiber loss can result from a variety of different issues and every optical fiber will have some degree of optical loss. The material of the optical fiber and its manufacturing tolerances are sources of optical fiber loss. Optical fiber loss can also be caused by unintended optical attenuation or discontinuity that occurs due to severe bending of the optical fiber or other damage to the optical fiber. Optical fiber loss can also result from discontinuities and alignment issues resulting from connectorizing optical fibers. For example, optical fiber communications networks include a number of optical interconnection points in fiber optic equipment and between fiber optic cables in which optical fibers must be interconnected via fiber optic connections. Each of these fiber optic connections provides for the possibility of a discontinuity or optical alignment issue.
FIG. 1 is a schematic diagram of an exemplary field optical fiber from a fiber optic cable that is prepared to be inserted into a housing of a fiber optic connector and to further be spliced to a stub optical fiber (referred to herein as a “stub fiber”) in the fiber optic connector to provide a joined optical fiber, wherein the field optical fiber and the stub fiber have dissimilar backscatter light collection efficiencies.
For example, FIG. 1 is a schematic diagram of an exemplary optical fiber 10 from a fiber optic cable 12 that is prepared to be inserted into a housing 14 of a fiber optic connector 16 to connectorize the optical fiber 10. The optical fiber 10 is exposed from a coating or buffer tube 18 to be prepared to be inserted into the housing 14. The optical fiber 10 of the fiber optic cable 12 can be connectorized in the field by a technician. In this example, the fiber optic connector 16 is from the UNICAM® family of fiber optic connectors available from Corning Cable Systems, LLC of Hickory, N.C. The fiber optic connector 16 contains a stub fiber 20 that is installed in a ferrule 22. The stub fiber 20 is shown in FIG. 1 extending from a front end 24 of the ferrule 22. U.S. Pat. Nos. 6,816,661 and 6,931,193, incorporated by reference herein in their entireties, describe a UNICAM® installation tool also available from Corning Cable Systems, LLC to splice the optical fiber 10 to the stub fiber 20 to connectorize the fiber optic cable 12. The splicing of the optical fiber 10 to the stub fiber 20 provides a joined optical fiber (not shown).
The technician may measure the loss in the optical fiber 10 after installation of the fiber optic connector 16 to ensure that the optical fiber 10 loss in the joined optical fiber comprised of the optical fiber 10 spliced with the stub fiber 20 is within acceptable limits. One method of measuring loss in an optical fiber is a double-ended optical loss measurement method. A double-ended optical loss measurement is a direct optical loss measurement. In a double-ended optical loss measurement, an optical signal from a light source is launched into one end of the optical fiber as an input end. A power meter coupled to the other end of the optical fiber as the output end measures the intensity of light transmitted through the optical fiber and received at the output end. The intensity of light at the output end of the optical fiber subtracted from the intensity of the light launched into the input end of the optical fiber is the amount of optical fiber loss. However, double-ended optical loss measurement requires access to both ends of the optical fiber. But, both ends of the optical fiber may not be accessible. Even if available, a double-ended optical loss measurement may require two technicians, one for each end of the optical fiber.
An optical time domain reflectometer (OTDR) can be used in a single-ended optical loss measurement method to indirectly measure optical fiber loss by only having access to one end of the optical fiber. In an OTDR, the light pulses from a light source are launched into an input end of the optical fiber. The optical fiber returns a small portion of light received at the input end back to the input end as scattered light due to the Rayleigh effect. This is referred to as “backscattered light.” The optical fiber has a backscatter light collection efficiency, which is based on the intensity of backscattered light in the optical fiber for a given intensity of light transmitted in the optical fiber in a forward direction. Thus, during the interval between the light pulses, this backscattered light from the optical fiber can be measured as an indirect method to measure the optical loss in the optical fiber. In an OTDR, a light detector is provided that receives the returned backscattered light from the optical fiber between the light pulses. An OTDR converts the detected backscattered light signal into an electrical signal, which is amplified, sampled, and displayed on an output device to indicate the optical fiber loss.
Determining any change in optical loss as a result of joining two or more optical fibers together using OTDR optical loss measurement would assume that the backscatter light collection efficiency of the optical fiber measured is identical before and after the joinder. This is necessary to be able to compare the optical loss before and after the joinder. However, this may not necessarily be true for certain types of optical fibers. For example, with reference to FIG. 1, the optical fiber 10 and the stub fiber 20 may have dissimilar properties that provide for each optical fiber to have different backscatter light collection efficiencies. Thus, the intensity of light backscattered by the optical fiber 10 and the stub fiber 20 for a given intensity of input light can vary. For example, the optical fiber 10 and the stub fiber 20 may have different core sizes. As another example, the optical fiber 10 and the stub fiber 20 may be manufactured from different materials and/or different manufacturers having different manufacturing processes. Even if the optical fiber 10 and the stub fiber 20 are manufactured by the same manufacturer using the same manufacturing process, variations in manufacturing processes can still occur that provide for the optical fiber 10 and the stub fiber 20 to have dissimilar properties.
Thus, an OTDR may not accurately measure optical fiber loss as a result of joined optical fibers when the loss event is due to the joinder of optical fibers having dissimilar backscatter light collection efficiencies. Determining any change in optical loss as a result of joining optical fibers using OTDR optical loss measurement would assume that the backscatter light collection efficiency of the optical fiber measured is identical before and after the joinder, so that the optical loss can be compared before and after the joinder.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinency of any cited documents.